The measurement of fluid velocity has been accomplished employing a variety of devices one of which is the magnetic flow meter. Magnetic flow meters are employed where the fluid whose velocity is to be measured is minimally conductive and employ the Faraday principle. In particular, a magnetic field is generated and sensing electrodes are provided to sense a voltage induced therein which can then be processed to obtain a measure of the fluid velocity. Magnetic flow meters come in two general forms; in one form, the magnetic field is generated by a magnet which surrounds the fluid flow, and in another arrangement, the fluid flow surrounds the magnet generating the magnetic field. The present invention deals with the latter type of flow meter, an example of which is described in my prior U.S. Pat. No. 3,855,858. This type of magnetic flow meter will be referred to in this application as a probe type flow meter.
While the voltages induced in the electrodes in the probe are accurate representations of the fluid velocity in the vicinity of the electrodes, the user normally extrapolates from the flow meter readings to obtain flow rates. For such flow rates to be accurate, the fluid velocity in the vicinity of the probe should give an accurate indication of the "average" flow velocity. One factor which will prevent accurate extrapolation is local perturbations in the fluid velocity in the vicinity of the probe. Such perturbations can arise from a variety of sources, at least one of which is the probe itself or the manner in which it is supported or mounted. In cases where the probe is in an environment in which the flowing medium is non-homogeneous, perturbations in the flow can arise as a result of the non-homogeneous nature of the medium. One particularly significant non-homogeneous medium in which flow meters may be used is waste water, sewage, or the like which may include debris.
In those cases where debris is generally of limited mass and rigid, while the debris may contact the flow meter and induce short term perturbations in the flow velocity readings, such debris will not usually introduce long term perturbations or errors. On the other hand, where the debris is non-rigid, for example, cloth, strings, or even partially dissolved particulate matter such as paper pulp, the debris may cling or "stick" to the probe. Under such circumstances, the velocity perturbations caused by such debris may well introduce longer lasting errors in the flow meter signals.
It is therefore an object of the present invention to provide a magnetic type flow meter which is arranged so as to tend to shed debris, even debris of the non-rigid type such as rags, cloth, strings, or "sticky" semi-dissolved matter. Since environments in which flow measurements are required, which include debris of the type mentioned above can exist in open channel flow or in pressure flow (such as in pipes) it is another object of the present invention to provide a magnetic flow meter of the probe type which is arranged to provide accurate flow measurements in flow of non-homogeneous media in either open channel or pressure flow.